Josh, you were the person I was hoping would answer since you've commented about this before and so I assume you have some experience with it 🙂 It sounds like it will be an overall slowing of our system:
One drawback to extending the archiving history time frame is that you will experience slower processing in your instance. Basically, with more history to search through for your lead database, Marketo will need more time to perform basic functions.
Thoughts?